Small-diameter carbon nanotubes having diameters between about 0.5 and about 3 nanometers, and lengths usually greater than about 50 nm, commonly known as “buckytubes,” have been the subject of intense research since their discovery due to their unique properties, including high strength, stiffness, thermal and electrical conductivity. The walls of small-diameter carbon nanotubes are fullerenes consisting essentially of sp2-hybridized carbon atoms typically arranged in hexagons and pentagons. Some small-diameter carbon nanotubes have only one wall, and others have more than one. Large-diameter multi-wall carbon nanotubes (MWNT), having diameters in excess of about 4 nanometers, are multiple nested carbon cylinders. Because large-diameter multi-wall carbon nanotubes have substantially greater density of defects in their side-walls, they are, consequently, mechanically less strong and electrically less conductive than small-diameter carbon nanotubes. Additionally, compared to the large-diameter multi-wall carbon nanotubes, small-diameter carbon nanotubes have considerably higher available surface area per gram of carbon.
Most methods for carbon nanotube production involve one or a combination of transition metal catalysts in contact with a carbon-containing feedstock at an elevated temperature, typically between about 700° C. and 1200° C. Some of the methods to make carbon nanotubes include electric arc, laser ablation of graphite, and gas phase techniques with supported and unsupported metal catalysts.
One method of preparing carbon nanotubes on supported metal catalyst is known as “chemical vapor deposition” or “CVD”. In this method, gaseous carbon-containing feedstock molecules react on nanometer-scale particles of catalytic metal supported on a substrate to form carbon nanotubes. This procedure has been used to produce large-diameter multi-wall carbon nanotubes, however, under certain reaction conditions, CVD methods can produce excellent single-wall and small-diameter multi-wall carbon nanotubes. Synthesis of small-diameter carbon nanotubes using CVD methodology has been described in Dai, et al. (1996), Chem. Phys. Lett., 260, p. 471-475, and “Catalytic Growth of Single-Wall Carbon Nanotubes from Metal Particles,” International Pat. Publ. WO 00/17102 A1, published Mar. 30, 2000, each incorporated herein by reference. The carbon nanotube material that results from a CVD process can comprise single-wall and/or multi-wall carbon nanotubes, residual catalyst metal particles, catalyst support material, and other extraneous carbon forms, which can be amorphous carbon, and non-tubular fullerenes. The term “extraneous carbon” will be used herein as any carbon that is not in the form of carbon nanotubes, and can include graphene sheets, graphite, non-tubular fullerenes, partial nanotube forms, amorphous carbon and other disordered carbon.
For many carbon nanotube applications, purified nanotube material is often desired. In the purification of a nanotube product made using a metallic catalyst on a support material such as a refractory oxide, removal of the support material, as well as the metallic catalyst residues and extraneous carbon, is desired.
Procedures for purification of carbon nanotubes are related in International Patent Publications “Process for Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO 02/064,869 published Aug. 22, 2002, and “Gas Phase Process for Purifying Single-Wall Carbon Nanotubes and Compositions Thereof,” WO 02/064,868 published Aug. 22, 2002, and incorporated herein in their entirety by reference. In addition to heating in an oxidative environment, both of these methods involve treatments to remove residual metal. In WO 02/064,869, metal is removed by an acid treatment in the liquid state, using such acids as hydrochloric acid, hydrofluoric acid, hydrobromic acid, hydroiodic acid, sulfuric acid, oleum, nitric acid, citric acid, oxalic acid, chlorosulfonic acid, phosphoric acid, trifluoromethane sulfonic acid, glacial acetic acid, monobasic organic acids, dibasic organic acids, and combinations thereof. In WO 02/064,868, metal is reacted with halogen-containing gases, such as chlorine, bromine, fluorine, and iodine, as well as their gaseous acids (HCl, HBr, HF, and HI) to form volatile metal compounds are removable by purging.
Many processing methods and articles of manufacture involving carbon nanotubes are enhanced by using a purified carbon nanotube material in which the presence of the catalytic metal and the catalyst support material is minimized. Purification procedures for removing the catalyst support and the catalytic metal that use strong acids, especially mineral acids, such as sulfuric acid and nitric acid, can damage to the nanotubes and cause loss of nanotube product. The ends of the nanotubes may be opened and existing defects in sidewalls enlarged. A purification method that minimizes such product damage and loss is desired. Additionally, implementing treatments with halogens and halogen acids, such as HF and HCl, poses greater expense, as well as scale-up difficulties, due to the chemical corrosivity that require special vessels, such as glass-lined vessels or vessels made of special metal alloys (e.g. Hastaloy C), additional process steps, and expensive chemical disposal. A method for purifying carbon nanotubes, and in particular carbon nanotubes made on a refractory oxide support, is sought, wherein the method is efficient, cost-effective and scalable.